viewing from computer to tv

i have bought a vga monitor cable to transfer pictures from my pc to the tv, i followed instructions on tv manual ( sony bravia ) but when i get to pc it says no signal? is this because i have a digi box connected to the aerial? what am i doing wrong?

Oldal, thank you for your advice, i am not computer minded, i am still not connected, i bought the cable which is a vga monitor cable hddb 15/mhddb15m: 10' sh. from Argos, the sales chap said this was what i needed to connect the computer to the tv, i attatched the cable to the tv and the pc,as shown on the Sony instructions, its from here i dont know what to do, i went through the tv remote through the AV channels until i got to the PC channel then it just shows (no signal) at the bottom of the tv.
are they supposed to just connect automatic or should i be doing something?also should i have the tv and the pc on when i connect the cable? am i supposed to go through something on the tv menu? or on the computer?
I hope you dont mind me asking.
aicogirl